A brushless motor generates an inverse electromotive force ωE (E: a magnetomotive force (main magnetic flux) of a permanent magnet used as a field magnet, ω: an axial angular velocity of the motor) when it is driven. Accordingly, an equivalent circuit for one phase of the brushless motor can be represented as shown in FIG. 11(a).
In FIG. 11(a), R is a per-phase primary resistance of the brushless motor, L is a per-phase inductance of the brushless motor, I is a primary current (phase current) of the brushless motor, and V is a terminal voltage applied to the brushless motor.
When driving the brushless motor by an inverter circuit, a value obtained by multiplying an input voltage of the inverter circuit by a voltage conversion ratio (output voltage/input voltage <1) of the inverter circuit is the terminal voltage V of the motor.
Further, when the terminal voltage V of the brushless motor is subjected to vector decomposition and expressed using a d axis voltage Vd and a q axis voltage Vq, the terminal voltage V is represented by following formulae (1) and (2).
                                          [                                                            Vd                                                                              Vq                                                      ]                    =                                                    [                                                                                                    R                                                                                                                                      ω                          ·                          Ld                                                                                                      ⁢                                                                                                                                          -                            ω                                                    ·                          Lq                                                                                                                                    R                                                                                            ]                            ⁡                              [                                                                            Id                                                                                                  Iq                                                                      ]                                      +                          [                                                                    0                                                                                                              ω                      ·                      E                                                                                  ]                                      ⁢                                  ⁢                                                      (        1        )                                V        =                                            Vd              2                        +                          Vq              2                                                          (        2        )            
Further, FIG. 11(b) is a vector diagram which is drawn on the basis of formula (1) considering that the primary resistance R is sufficiently small.
In FIG. 11(b), Ld is a d axis inductance, Lq is a q axis inductance, Id is a d axis current (field current), and Iq is a q axis current (torque current). The field current Id and the torque current Iq are represented by formulae (3a) and (3b) as follows.Id=Ip·sin β  (3a)Iq=Ip·cos β  (3b)wherein β is an angle of advance (advance angle) in the phase of the current that flows into the motor (motor current) with respect to the rotor position of the brushless motor, and Ip is the amplitude of the motor current I.
The above-mentioned formulae (1) and (2) indicate that vector control for the brushless motor, i.e., motor control using the field current Id and the torque current Iq, is possible. More specifically, the above-mentioned vector control is to vary a command value of the torque current Iq on the basis of an output torque that is required of the brushless motor, while controlling a command value of the field current Id so as to be a constant value (for example, 0). By controlling the inverter circuit for driving the brushless motor on the basis of these command values, an output torque T represented by formula (4) can be obtained.T=E·Iq+(Ld−Lq)·Id·Iq  (4)
The first term of formula (4) indicates a torque component generated by a permanent magnet as a field magnet, i.e., a magnet torque, and the second term indicates a reluctance torque caused by saliency of the brushless motor. Accordingly, when the brushless motor is a non-salient pole motor, Ld=Lq holds, and only the first term remains in formula (4). When the brushless motor is a salient pole motor, Ld≠Lq holds, and the second term of formula (4) has a value.
Further, the terminal voltage V of the motor is represented by formula (5) as follows.V=ω·E+j·ωLq·Iq+j·ω·Ld·Id  (5)
As can be seen from formula (5), as the rpm of the brushless motor, i.e., the axial angular velocity ω of the brushless motor, increases, the counter-electromotive voltage ωE increases in proportion to the axial angular velocity ω. Accordingly, if the increase in the counter-electromotive voltage ωE is allowed as it is, the terminal voltage V of the brushless motor becomes higher than the input voltage of the inverter circuit due to the increase in the counter-electromotive voltage ωE, resulting in a problem that the brushless motor cannot be driven at the higher rpm.
As a solution of the above-mentioned problem, there is a method called “weak field control” (for example, refer to Japanese Patent No. 3146791 (FIGS. 1 and 10).
In this method, the motor terminal voltage V in the high rpm area can be reduced to no more than the input voltage of the inverter circuit, by supplying a field current Id and performing control for generating a field magnetomotive force that weakens the field magnetomotive force of the permanent magnet. The field current Id having such characteristics is called a weak field current. The weak field current Id is predetermined from the motor rpm N and the torque T. To be specific, the correspondences between the values of motor rpm N and torque T, and the values of weak field current Id suited thereto are defined on a table (map) or the like. Under the actual control of the field current Id, the weak field current Id is set at a value suited to the corresponding torque T and rpm N, using the above-mentioned table (for example, refer to “No. 74, Rpm Control System using Weak Magnetic Flux Control of PM Motor”, International Symposium of Industry Applications Society of the Institute of Electrical Engineers of Japan).
However, when controlling the weak field current Id using the table values as described above, the weak field current Id becomes excessively large or small with variations in the input voltage of the inverter circuit. As a result, the motor driving efficiency is reduced, or the required torque is not satisfied and thereby the maximum rpm cannot be realized.
For example, when the inverter input voltage is high, a weak field current Id larger than required is supplied. As is evident from the vector diagram shown in FIG. 11(b) and formula (5), although the terminal voltage is lowered when the weak field current Id flows, a current Id that does not contribute to occurrence of torque is generated, leading to a reduction in efficiency.
Conversely, when the inverter input voltage is low, a weak field current Id that is sufficient to reduce the motor terminal voltage V to no more than the inverter input voltage cannot be supplied, and further, a torque current Iq that is sufficient to obtain a required torque cannot be supplied.
In order to solve the above-mentioned problems, there has been proposed a method including detecting an inverter input voltage, and calculating a weak field current on the basis of the detected voltage and a torque required of a motor (for example, refer to Japanese Patent No. 3146791 (FIGS. 1 and 10).
Further, as a method for determining a weak field current Id, there has been proposed a method including detecting that an inverter output voltage becomes equal to or larger than an inverter input voltage, and controlling a weak field current Id so as to resolve such state where the inverter output voltage is high (for example, refer to Japanese Published Patent Application No. 2000-341991 (FIG. 1)).
However, in the conventional motor driving apparatus that controls the weak field current of the motor according to the input voltage of the inverter circuit, when the input voltage of the inverter circuit varies steeply or periodically, the command value of the weak field current varies according to variations in the input voltage. In other words, there is a fear of extremely unstable behavior of the motor depending on the shapes of variations in the input voltage.
Further, since the conventional motor driving apparatus has a circuit for detecting the inverter input voltage, detection accuracy and responsivity in this circuit may adversely affect determination of a weak field current that is a control variable in weak field control.